The present invention relates to a compact, flat antenna for a circularly-polarized wave which is suitable for use in surface mounting. More particularly, the present invention relates to a compact plane antenna for a circularly-polarized wave which enables transmission and reception of a circularly-polarized wave without use of a special feeding circuit.
A dominant-mode patch antenna is often used as a flat antenna of this type for a circularly-polarized wave. As shown in FIGS. 7A and 7B, the antenna of this structure comprises a ceramic substrate 21 serving as a dielectric substrate, and a patch antenna element 22 provided on the surface of the ceramic substrate 21. Further, a ground conductor 23 is provided on the side of the ceramic substrate 21 opposite to the side where the patch antenna element 22 is disposed. A feeding pin 24 is connected to a feeding section 22a provided on the reverse side of the patch antenna element 22, by way of a through hole formed in the ceramic substrate 21 and that formed in the ground conductor 23. In principle, in the dominant-mode patch antenna, two sides A, which are orthogonal to each other within a plane, must be formed to an electrical length of substantially xc2xd wavelength. In order to make the dominant-mode patch antenna compact, a dielectric substrate having a large dielectric constant must be used as the dielectric substrate 21. Provided that the relative dielectric constant of the dielectric substrate 21 is taken as xcex5r, the length of one side A can be shortened in proportion to xcex5rxe2x88x92xc2xd. For example, a GPS vehicle-mounted receiving terminal has been reduced to about one-fifth the size of a receiving terminal which is embodied without use of a substrate of high dielectric constant (one side is reduced to a size of about 20 to 25 mm).
However, in applications involving use of a small communications device, such as a portable receiving terminal, demand has existed for a smaller and less weight antenna whose size is about one-quarter that of a recently-available antenna (which has a side of about 5 mm and is one-twentieth the size of an antenna not provided with a substrate having a high dielectric constant). Even when an attempt is made to embody such an antenna by means of increasing the dielectric constant of the substrate in the manner as mentioned above, difficulty is encountered in matching a resonance frequency, due to limitations imposed by manufacturing technology. Producing a high-dielectric substrate having xcex5r of 100 or more from low-loss material is difficult, because of limitations imposed by material.
An inverted-F-type antenna has hitherto been known as a related technique for miniaturizing a flat antenna for a linearly-polarized wave. The inverted-F-type antenna is formed as follows: Of two segments of an L-shaped conductive line (or plate), an open end of a shorter segment is grounded, and a longer segment is situated in parallel with the ground. Further, a feeding segment serving as a third conductor is placed substantially in parallel with and spaced apart from the shorter segment so as to satisfy requirements for impedance matching, as required. The feeding segment is formed from a conductive line (or plate) for connecting a power feeding point and the longer segment.
As shown in FIGS. 8A and 8B, a conductive film is formed so as to extend from one surface to a side surface of the substrate 21, thereby forming an L-shaped conductor (radiator element 22). The open end of the conductive film provided on the side surface (serving as a shorter segment) is connected to a ground conductor 23 provided on the reverse side of the substrate 21. A feeding pin 24 is connected to a feeding section 22a of the radiator element 22 by way of a through hole formed in the substrate 21 and that formed in the ground conductor 23. In this structure, a longer side B of the conductive film (i.e., the radiator element 22) provided on one surface of the substrate 21 can in principle be formed so as to assume an electrical length of substantially xc2xc wavelength. The F-shaped antenna can be reduced to half the length of the dominant-mode patch antenna.
As mentioned above, a very compact antenna for a circularly-polarized wave required for a recent portable terminal, such as a small communications device, cannot be formed from the dominant-mode patch antenna.
On the other hand, the inverted-F-type antenna is to be used for a linearly-polarized wave. If an attempt is made to construct an antenna for a circularly-polarized wave from an inverted-F-type antenna, two independent inverted-F-type antenna elements must be provided orthogonal to each other within a plane. Furthers there must be employed a special feeding circuit, such as a 3dB hybrid link, for making the amplitudes of feed signals to be sent to the antenna elements equal, as welt as for making the phase of one feed signal orthogonal to that of the other feed signal. As a result, the antenna cannot be made compact, which in turn imposes an impediment to productivity.
The present invention has been conceived to solve the drawbacks of the related antennas, and it is therefore an object of the invention to provide a very compact antenna for a circularly-polarized wave which can be mounted on a compact portable terminal, such as a compact communications device.
Further, it is another object of the invention to provide a compact antenna for a circularly-polarized wave to be used with a compact communications device, the antenna being able to transmit and receive both a right-handed circularly-polarized wave and a left-handed circularly-polarized wave.
In order to achieve the above objects, according to the present invention, there is provided an antenna for a circularly-polarized wave comprising:
a ground conductor plane;
an excitation electrode provided substantially in parallel with the ground conductor plane;
a pair of electrodes for radiating a linearly-polarized wave which are provided substantially in parallel with the ground conductor plane, with the excitation electrode interposed therebetween; and
a feeding section electrically connected to the excitation electrode,
wherein first ends of the respective radiation electrodes oppose to the excitation electrode, thereby constituting capacitive coupling; and wherein second ends of the respective radiation electrodes are connected to the ground conductor plane such that the directions in which electric fields are to be excited become substantially orthogonal to each other.
In this configuration, two electrodes for respectively radiating a linearly-polarized wave are constructed so as to cause excitation independently of the excitation electrode in a non-contact manner. Hence, the two radiation electrodes can be simultaneously excited so as to be electrically independent of each other. Hence, the compact antenna can transmit and receive a circularly-polarized wave without involvement of equal distribution of power or without use of a special feeding circuit for realizing a 90-degrees phase shift.
Preferably, an electrical length of each of the radiation electrodes should be substantially quarter wavelength of a desired frequency band. If each of the radiation electrodes is provided so as to extend from the first end to the second end in a meandering manner, a longer electrical length per unit dimension can be ensured, thereby rendering the radiation electrodes more compact.
Alternatively, each of the radiation electrodes extending from the first end may be folded at least once toward the first end so as to be able to cause resonation at two frequency bands. In this configuration, the antenna can transmit and receive signals of two frequency bands: that is, a signal of first frequency and a signal of second frequency which is about double the first frequency band.
Preferably, a dielectric member is interposed between the ground conductor plane, the excitation electrode, and the radiation electrodes. If the dielectric constant of the dielectric member is increased, the radiation electrodes can be made more compact.
Here, it is preferable that the dielectric member is a dielectric substrate. The excitation electrode and the radiation electrodes are provided on a first surface of the dielectric substrate, and the ground conductor plane and the feeding section are provided on a second surface of the dielectric substrate parallel to the first surface. A conductive film for electrically connecting the second ends of the radiation electrodes and the ground conductor plane, and a conductive film for electrically connecting the excitation electrode and the feeding section are formed on the side surfaces of the dielectric substrate. In this configuration, the antenna can be produced readily, and use of a substrate having a larger dielectric constant enables a further reduction in the size of the dielectric substrate.
Here, it is preferable that the excitation electrode has two equal-length sides orthogonal to each other. Each of the radiation electrodes is a strip-shaped member formed with a shorter side section constituting the first and second ends and a longer side section. The shorter side sections at the first ends of the respective radiation electrodes oppose to the two equal-length sides of the excitation electrode substantially in parallel with each other.
Alternatively, it is preferable that the excitation electrode is a conductor piece having two opposing sides which are substantially parallel with each other. Each of the radiation electrodes is a strip-shaped member formed with a shorter side section constituting the first and second ends and a longer side section. The shorter sides of the first ends oppose to substantially in parallel with the two opposing sides of the excitation electrode while making an angle of 45 degrees with respect to the respective longer sides.
Here, the xe2x80x9cstrip-shaped memberxe2x80x9d means a conductor piece having a side opposing the excitation electrode and sides longer than the side. The strip-shaped member includes a conductor piece which is folded into a zigzag pattern in the direction perpendicular to the longer sides (or the direction in which the conductor piece is to extend), and a conductor piece folded in the direction parallel to the direction in which the conductor pieces is to be extended. Further, the strip-shaped member can include a plate-shaped conductor piece, a conductive film, and a conductor line.
Preferably, the dielectric substrate is formed so as to assume a first arm section and a second arm section, which are orthogonal to each other. The pair of radiation electrodes are provided on at least respective first and second arm sections. In this configuration, the dielectric substrate can be made more compact.
Preferably, the antenna further comprises a second pair of electrodes for radiating a second linearly-polarized wave. One of the two pairs of electrodes is used for a right-handed circularly-polarized wave, and the other is used for a left-handed circularly-polarized wave.
Preferably, the excitation electrode is a conductor piece having two opposing sides which are substantially parallel with each other at a comer of the dielectric substrate. A corner portion of a side surface of the dielectric substrate is chamfered in a direction orthogonal to the two opposing sides of the excitation electrode. The conductive film is formed in the chamfered section.